In mining for recoverable minerals, blasting provides the first step in breaking and dislodging the host rock from its initial state in the ground. This is the case whether the mining is conducted largely as a surface, or open-cut operation, or largely as a subsurface, or underground, mining operation. Blasting for recoverable minerals may occur either in rock that largely comprises waste or overburden material or in rock comprising ore or other recoverable mineral which represents recoverable concentrations of the valuable mineral or minerals to be mined. In some cases, blasts may occur in both waste and recoverable mineral.
Mine productivity can be improved through blasting which achieves more effective breakage and/or movement of the rock. This may improve the efficiency of mining equipment such as excavators and haulage or conveying equipment. Furthermore, in the case of mining for metalliferous mineral, improved rock breakage may lead to improvements in performance and throughput of the downstream comminution and ore recovery processes. In particular, finer fragmentation may improve performance and throughput of the crushing and milling circuits, which are generally the most cost- and energy-intensive stages of rock processing for ore recovery. In addition to the physical size of the rock fragments, it is believed that weakening of the inherent structural strength of the rock may further improve crushing and grinding performance. The creation of macro- and micro-fractures in the blasting process is thus believed to contribute to such improved comminution performance.
Mine-to-mill studies have shown that modest increases, of the order of 10-20%, in explosives powder factor can deliver increased milling throughput. It has been proposed that more dramatic increases, of the order of a factor of 2-10, may actually result in explosives energy performing much of the comminution process and lead to much larger increases in mill throughput. The economic impact of even a 10% increase in mill throughput is enormous for many metalliferous or precious metal mines. Additional benefits will flow from reductions in electricity consumption and the associated greenhouse gas emissions, which can also have an economic value attached to them.
Up to now the major constraints on achieving very high explosive energy concentrations in blasts, which are conventionally expressed in terms of powder factors, have been largely around control of the increased energy. Blast designs need to safely contain the explosive energy to avoid flyrock, excessive vibration and noise, and damage to surrounding mine infrastructure, including highwalls or remaining intact rock. In underground mining, rock breakage is sometimes intended to be limited to the zones of ore, for example within stopes, without unduly breaking waste rock around the ore zone. If waste rock is broken into the stope then the ore-to-waste ratio decreases; a deleterious process known as dilution. Also excessive damage to surrounding rock may lead to mine instability. Access tunnels, or drives, also need to be protected from excessive damage.
Increases in explosives energy or powder factor have thus generally been restricted by these factors. Where blast designers have strived to maximize explosive energy within the blast to achieve improved fragmentation, the blast designs have generally been limited to the highest powder factors that avoid flyrock and other damaging environmental incidents.
It would thus be a major advantage in mining if blasting could effect improved fragmentation and fracturing of rock that requires comminution. The present invention provides such an improvement while simultaneously ensuring that deleterious blast environmental effects are safely constrained.
As noted above, blast designers conventionally describe the explosives energy concentration within blasts by the powder factor. Powder factors are typically expressed in terms of the explosive mass per unit of unblasted rock volume or mass. Thus powder factors may be expressed as kilograms of explosive per bank, or solid, cubic meter of unblasted rock (kg/bcm or kg/m3). Powder factors may also be expressed as kilograms per tonne of unblasted rock (kg/t). Rarely, powder factors may be expressed in terms of volume of explosive per unit volume or mass or rock. Other units, such as Imperial units of pounds of explosive per cubic foot of unblasted rock (lb/ft3) or even mixed units such as pounds of explosive per tonne of rock are also used. Occasionally, where the explosives energy content per unit mass is known, blast designers may express powder factors in terms of explosive energy per unit rock volume or mass, such as for example MJ of explosive energy per tonne of unblasted rock (MJ/t rock). It is to be understood that while metric units of explosive mass per unit volume of unblasted rock are used here, all such systems of units may be used interchangeably by simply applying the appropriate unit conversion factors, density or explosive energy content per unit mass.
Conventionally, global blast powder factors describe the total mass of explosive in the blast field divided by the total rock volume or mass in the blast field. However, localized powder factors may also be used to describe powder factors in regions or zones of blasts. In such cases, a zone may be defined by the blast designer as a region within certain geometrical points, lines, planes or surfaces within the blast. Blast limits or perimeters are usually defined by the outermost blastholes or free surfaces or edges. Occasionally, an additional amount of rock may be added to the outermost holes to define the blast field or zones therein. Such an additional amount may constitute a fraction of the burden or spacing of the outermost blastholes. Such limits may also define the perimeters of blast regions or zones. The ends of columns of explosives, or interfaces with inert stemming material, may also conveniently be used as points for defining blast zones or layers. At the level of individual holes, powder factors may be expressed as the explosive content (mass or energy) per unit of rock volume surrounding the hole, that is the rock volume that the specific hole is intended to fracture in the blast. Conventionally thus, the powder factor can also be expressed as the explosive content in the hole (mass or energy) divided by the product of the hole burden, spacing and depth (or the total height of the blast zone). The rock volumes thus calculated may also be converted to rock mass by multiplying by the rock density, where it is desired to express powder factor in terms of explosive mass per unit mass of rock. Where blastholes patterns and explosive loading in the blastholes are regular through the blast field, the global blast powder factor will equal localised or even individual blasthole powder factors.
Powders factors used in common blasting techniques, both in open cut and underground mining for recoverable mineral, are generally of the order of 1 kg/m3 or less for production blasts. Examples, definitions and calculations of powder factors and conventional blasting methods may be found in:                ICI Handbook of Blasting Tables, July 1990;        Orica Explosives Blasting Guide, August 1999, ISBN 0 646 24001 3;        ICI Explosives Safe and Efficient Blasting in Open Cut Mines, 1997; and        Tamrock Handbook of Surface Drilling and Blasting.        
Examples of powder factors in a Stratablast® blasting technique of Orica Mining Services, Australia are given in WO 2005/052499.
Occasionally powder factors may be increased to about 1.5 kg/m3, and there have also been reports of the use of powder factors as high as 2.2 kg/m3 in some open cut mines. Such high powder factors have been used rarely in production blasting, for very hard rock, with the hardness of the rock and the adjustment of stemming being used to control flyrock.
In special blasting circumstances in underground mining, powder factors may be higher than this. However these circumstances have been in the construction of shafts, access tunnels or drives, or so-called rises, raises, slots or ore passes to provide conduits for transporting broken ore. These situations comprise blasts in highly confined spaces where dilution of ore is not an issue. By contrast, blasting of ore for recoverable mineral in stopes is conventionally performed at powder factors below 1.5 kg/m3 in order not to excessively damage surrounding intact rock or mine structure or cause excessive dilution of the ore by breaking surrounding waste rock into the ore.